Technical Field
The present invention relates to a radiation image processing apparatus and method that performs image processing to correct contrast or frequency characteristics of a radiation image such as, for example, a breast image.
Background Art
Image diagnosis through the use of a radiation image capturing system (called mammography) for imaging a breast has been drawing wide attention to facilitate early detection of breast cancers. A radiation image of a breast (breast image) captured by the mammography is subjected to image processing in a dedicated operation terminal or the like, and used by a doctor for diagnosis. The doctor checks for a lesion such as, for example, a tumor mass or a calcification by displaying the breast image on a display and interpreting the image.
The image processing described above is performed for ease of interpretation of the breast image. More specifically, image processing conditions for performing image processing are determined based on the breast image such that image characteristics, including density, gradation, dynamic range, frequency characteristics, and noise of a target interpretation area, become appropriate to obtain a breast image having a desired image quality, and image processing is performed according to the determined image processing conditions.
In the meantime, in capturing a radiation image, exposure dose of the subject by X-ray is preferably reduced as much as possible. Further, in order to obtain a radiation image that allows an appropriate interpretation diagnosis, X-ray having an energy spectrum corresponding to the X-ray absorption property of the imaging region needs to be applied to the subject.
For example, in the aforementioned mammography, a diseased tissue having a very small difference in X-ray absorption amount with respect to a normal tissue is extracted in high contrast, so that low energy X-rays are typically used. Further, in applying an X-ray to a subject, the use of characteristic X-ray is efficient and a target that generates an X-ray when hit by an electron beam is selected according to the X-ray absorption property of the imaging region for that purpose. In the meantime, the increase in high energy X-ray component may reduce the contrast of a radiation image while the increase in low energy component may increase the exposure dose of the subject. Thus, a filter that can selectively absorb a high energy X-ray component or a low energy X-ray component is selected according to the X-ray absorption property of the imaging region.
Commonly used target types include Mo (molybdenum), Rh (rhodium), and W (tungsten), while commonly used filter types include Mo (molybdenum) and Rh (rhodium). The use of Mo in both the target and the filter (Mo/Mo) may result in a relatively large amount of low energy X-ray component and a high contrast image quality, but the exposure dose of the subject is increased. On the other hand, the use of W in the target and Rh in the filter (W/Rh) may result in a relatively large amount of high energy X-ray component and a low contrast image quality, but the exposure dose of the subject is reduced. The radiographer sets radiation application conditions such that a desired image quality is obtained according to the subject while reducing the exposure dose as much as possible.
As described above, image processing is performed on a radiation image obtained by an imaging system. Performance of the same image processing regardless of the types of the target and the filter used, however, the image qualities may differ, which may affect the diagnosis based on the obtained radiation image. For example, a radiation image obtained by the combination of W/Rh is reduced in contrast due to a reduced density range of the image with respect to a radiation image obtained by the combination of Mo/Mo as described above. Such a trouble is a large problem when changing the target or the filter while obtaining radiation images of left and right breasts of the same subject or performing imaging by changing the imaging directions.
As such, a method is proposed that obtains an appropriate radiation image by selecting image processing conditions, including gradation processing conditions and frequency processing conditions according to X-ray application conditions, such as the type of target, the type of filter, and the tube voltage, without depending on the radiation quality (Japanese Unexamined Patent Publication No. 2010-131179).
In the meantime, parameters for gradation processing and frequency processing differ depending on the thickness of the subject. For example, the transmission amount of X-ray differs between a thick portion and a thin portion of a subject, resulting in different contrasts. For this reason, a method that measures the thickness of a subject and sets a parameter for emphasizing the contrast by performing gradation processing according to the X-ray exposure amount and the thickness (Japanese Unexamined Patent Publication No. 2012-100734), and a method that sets a parameter for frequency processing according to X-ray application conditions and a body thickness (Japanese Unexamined Patent Publication No. 2011-239804) are proposed.